In order to clarify the effect of tritium(³H or T)on both the ecosystem and the reactivity of compounds including H atom(s), the hydrogen-isotope exchange reaction (T-for-H exchange reaction) between each benzaldehyde derivative and tritium-labeled poly (vinyl alcohol) was observed in the range of 50 to 90°C. As a result, the radioactivity of the compounds increased with the reaction time. Applying the A"-McKay plot method to the observed data, the rate constants(k) of the functional group in each compound were obtained. Comparing these k with each other, the following six matters have been clarified in the reaction. (1)The reactivity of benzaldehyde derivatives depends on temperature. (2)The order of the temperature dependence in the reactivity of the fluorobenzaldehyde tends to o-substitute > m-substitute > p-substitute, and that in the reactivity of the chlorobenzaldehyde tends to m-substitute > o-substitute > p-substitute. (3)The order of reactivity in each chlorobenzaldehyde derivatives tends to o-position > m-position > p-position. (4)The reactivity of the formyl group in the benzaldehyde derivative can be estimated by applying both the A"-McKay plot method and the Hammett rule. (5)The method used in this work is useful for evaluating the influence of T on environment.